A discharge lamp of the general type to which the present invention relates is described in U.S. Pat. No. 4,415,829. This lamp is intended to be used with direct current operation, and has a power rating of less than 100 W. to insure that the discharge arc will start at the cathode tip, a winding or twist is located close to the end point of the electrode forming the cathode.
Single-based or single-ended discharge lamps of this type are sometimes difficult to start rapidly and reliably when cold; after having been operated for some time, and when the lamp is hot, re-starting may be even more difficult. Slow or difficult starting, resulting apparently from poor transfer of the arc on the electrode, also interferes, in due course, with light output since the discharge vessel or arc tube or arc vessel will become blackened.
It appears that the poor transfer of the arc to the tips of the electrodes may be due to slow heating of the electrodes to operating temperature. Such electrodes frequently have substantial thermal losses. The heating time of the electrodes is proportional to the square of the radius of the electrode wire, and inversely proportional to the length of the electrodes and to the current passing through the lamp. The electrode wires--typically of circular cross section--cannot be dimensioned at random; if one starts, for example, form a fixed radius of the electrode wire and a predetermined fixed operating current of the lamp, decreasing the heating time can be obtained only by increasing the length of the electrode. This, however, then interferes with a desired miniaturization of the lamp.
German Patent Disclosure Document DE-OS No. 32 42 840 describes an electrode which is formed of a straight shaft portion, close to the path or press seal, and a second portion formed in wound or coil form close to the discharge end, in which the coils of windings are so arranged that adjacent windings of the coil portion do not touch each other. In such an electrode, the discharge arc can start at any place at the last winding, facing the discharge region, of the coiled portion. This, then, results in formation of the discharge arc within the discharge vessel at a random position. Consequently, such a lamp cannot be used when intended for combination with an optical system where the discharge must be placed, accurately, at a focal point or other predetermined defined point with respect to a reflector or to a lensetic system, for short in an optical system, where the position of the arc must be predetermined.